Printing a digital document is one of the most efficient ways to convey information to a user. New print-on-demand technologies such as laser printing and inkjet printing enable to print documents almost instantaneously without the need for creating intermediate printing masters.
Inkjet printing works by jetting ink droplets through a nozzle onto a substrate.
In the case of continuous inkjet, a continuous stream of electrically charged ink droplets is produced and electromagnetic fields are used to guide this stream away from or towards a substrate as to form an image on said substrate.
In the case of drop-on-demand inkjet, a mechanical or thermal energy pulse is applied to ink residing in a small chamber in order to create a pressure wave that propels a miniscule ink droplet at high speed through the nozzle towards a substrate. The pressure wave is controlled by shaping the length and the profile of the electrical waveform that is applied to the thermal or mechanical transducer in the ink chamber. In many cases the volume of the droplet and the size of the ink spot are substantially fixed. In other cases, the volume of the droplet can be modulated to create ink spots having different sizes on the substrate.
Printing the image of a document is usually achieved by moving the nozzle relative to the substrate along a raster by means of a shuttle in combination with a substrate transport mechanism and selectively jetting ink droplets on a substrate in response to the image of said document.
When the ink droplets land on a substrate, they form ink spots. Because these ink spots are small, they cannot be individually resolved by the human visual system but together they render a visual impression of the image of the printed document. Generally a halftoning technique is used to determine the spatial distribution of ink spots that produces an optimal rendering of the image of a given document.
A well know halftoning technique is error diffusion and is explained in the book “Digital Halftoning” by Robert Ulichney, published by the MIT Press, Cambridge Mass. In this book, different variations of the error diffusion technique are presented that can be used to convert a two-dimensional image of pixels represented on a substantially continuous tone scale into a halftoned image of pixels that can take only two tone values corresponding with an ink spot or no ink spot. Many other variations on the error diffusion technique have been described, including multilevel error diffusion techniques to drive printing devices capable to render more than two tone values, vector error diffusion to render color images and error diffusion techniques that produce a more homogeneous distribution of halftone dots in the highlight regions of an image.
To increase printing speed usually not one but an array of nbrNozzles inkjet nozzles are generally used that can be operated in parallel. Such an array of nozzles makes up a print head.
By moving a shuttle comprising a print head across the substrate in a fast scan orientation, a set of parallel raster lines of pixels can be printed in one step. Such a set of raster lines is called a swath.
When a swath has been printed, the print head is moved in a slow scan direction over a distance of the length of the array of nozzles to print an additional swath of lines underneath said previous swath. This process of printing swaths is repeated until a complete document is printed on the substrate.
The smallest value for the nozzle pitch is practically limited by constraints imposed by the manufacturing process. For reasons of image quality, however, a printing pitch in the slow scan direction is often desired that is smaller than the nozzle pitch. The document U.S. Pat. No. 4,198,642 teaches that a value can be selected for the printing pitch in the slow scan orientation that is an integer fraction 1/n of the nozzle pitch by using an interlacing technique.
Because of manufacturing tolerances, systematic variations between nozzles belonging to the same inkjet head exist of the volume of droplets and of both their ejection velocity and direction. If all the ink droplets of a single line of pixels in the fast-scan orientation are printed by the same nozzle, the variations in the ejection direction across the slow-scan orientation show up as correlated image artifacts that look like banding or streaking.
The document U.S. Pat. No. 4,967,203 introduces a technique to resolve this problem. By having the pixels on one and the same line printed by different nozzles instead of by the same nozzle, the correlated image quality artifacts can be de-correlated. The underlying assumption is that the image quality artifacts caused by variations between different nozzles are uncorrelated. De-correlating the image quality artifacts diffuses them over the printed substrate so that they become less perceptible or preferably imperceptible. In many documents, this technique is referred to as shingling. The method presented in U.S. Pat. No. 4,967,203 reduces printing speed, because multiple passes of the print head are required to print all the pixels on a line.
In U.S. Pat. No. 6,679,583 an improved technique is presented that combines the effects of the teachings in U.S. Pat. No. 4,198,642 and U.S. Pat. No. 4,967,203 and adds a number of other improvements, including improved printing speed. In this document, the term mutually interstitial printing is introduced to describe both interlacing and shingling. The term mutually interstitial printing also avoids confusion, as the term shingling is preferably used in the graphic arts industry to describe a technique that compensates for the effects of the thickness of the paper on the width of the margin in saddle-stitched bookmaking.
Once an ink droplet ejected by a nozzle lands on a substrate, it is being cured so that it receives the required resistance against rubbing. Ink curing can be achieved by a number of mechanisms.
A first mechanism of ink curing is absorption of the ink into fibers of the substrate or a porous coating. This is the dominant mechanism when oil or water based inks are used.
A second mechanism of ink curing is coagulation of the ink by evaporation of an ink solvent. When the ink solvent has evaporated, pigments or dyes together with a binder material are left on the paper.
In many practical applications, a combination of the two above effects takes place: ink is initially absorbed by a substrate and then, depending on the vapor pressure of the solvent, evaporates in a shorter or longer time.
A third mechanism of ink curing is polymerization, for example under the influence of an external energy source such as a UV light source. The high-energy radiation creates free radicals that initiate a polymerization reaction that solidifies the ink. The main advantage of this technique is that it enables the printing on media that do not absorb ink.
A fourth mechanism of ink curing is phase or viscosity change by temperature. Ink is jetted at a high temperature when it is in liquid phase, and solidifies when it cools down on the printed surface.
Especially in industrial print applications, such a poster printing, textile printing, decoration printing, packaging printing etc, there is a demand to print large areas in a short time.
A first approach to meet this demand is to increase the number of nozzles of a print head. Printing with more nozzles in parallel directly increases printing performance. The company XAAR plc., located in Cambridge UK, has demonstrated print heads with a length of 30.5 cm (12 inches) that print with 142 nozzles per cm (360 nozzles per inch). However, because the chance that a head contains a nozzle that is defective during production or that becomes defective during use increases more than proportionally with the length of the head, the production yield and reliability of such long heads tends to be low. This results in high manufacturing and maintenance costs. Another problem is that for certain applications even longer heads are needed. Besides that this would drive manufacturing and maintenance costs even further up, no tooling equipment actually exists for manufacturing such heads.
A second approach to increase the number of nozzles that can print in parallel and that was successfully taken by the company Agfa Dotrix N.V. located in Ghent Belgium, was to use an assembly of a plurality of smaller print heads. By mounting the heads on a print head mount, a print head assembly is obtained that has at least the same width or length as a document that has to be printed. This implies that such a document can be printed in one single swath.
The above approach is extremely effective for maximizing printing performance, not only because of the high degree of parallel printing by multiple nozzles but also because it makes the movement of a print head along slow scan orientation obsolete. It also addresses the problem of reliability and manufacturing and maintenance costs associated with the use of long print heads. However, it does introduce a number of other problems.
It was mentioned already that systematic variations between nozzles belonging to the same or different inkjet heads exist of the volume of droplets and of both their ejection velocity and direction. The causes of these variations are divers, but all come down to the effects of tolerances in different steps of the manufacturing such as the nozzle plate etching, the shaping and the mounting of the piezoelectric component and the back-to-back mounting of two print head in a sub-assembly.
One effect is that the diameter and shape of the nozzles on the nozzle plate may vary. A second effect is that the efficiency of the actuators in the ink chambers of different nozzles may vary, for example because of uneven stress on the piezoelectric material of the transducer or because of a non-uniform force sensitivity of the piezoelectric material. A third effect is that the glue layer thickness between the piezoelectric material and the other materials of the ink chamber may vary.
In conventional ink jet systems, the effects of these variations on image quality are managed by using mutual interstitial printing. Mutual interstitial printing relies on the printing the pixels on one single line in several passes and by different nozzles. This approach, however, is not compatible with concept of single pass printing in the above system.
A possible solution to correct for uneven droplet volume would be to adjust the individual voltages that drive the actuators. This, unfortunately, is currently not possible since existing driver electronics of a print head simply do not offer this option.
A second solution would be to make a profile of the density variations in an image and to correct for these variations in the image of the document that is to be printed prior to the halftoning step. A disadvantage of this approach is that the corrected image cannot be repurposed since it is suitable for printing only on the device and specifically at the position on the device for which it was corrected.
A third solution would be to introduce a certain amount of noise to the image pixel values prior or past the halftoning step. Introducing noise can be an effective manner to mask correlated image artifacts that result from uneven droplet volumes. A disadvantage of this method, however, is that it tends to make the printed result grainy, which is undesirable from an image quality viewpoint.
The document US2002/0181987 describes an image processing method and a printing control system for an inkjet printer. The system accepts a color image, typically represented in an RGB color space, and first performs a spatial resolution conversion step to obtain a color image represented at the spatial resolution of the printing system [0085]. In a following step, every pixel in the color image is separated into a pixel represented in a colorant space, for example a cyan, magenta, yellow and black colorant space [0086]. A next step consists of separating for every pixel the magnitude of every colorant into a mixture of magnitudes corresponding to the droplet volumes that the printer can physically render. This step produces for every pixel and for every droplet volume of every colorant a magnitude represented on a near continuous scale [0088]. A final step consists of halftoning said near continuous magnitudes into a spatial distribution of binary dots for every droplet volume of every colorant. This step yields a printable binary image for every droplet volume that can be rendered with every color on said inkjet printer [0095]. The above referenced document also teaches an additional image processing step after the halftoning in which combinations of dots having droplet volumes on a given pixel location are replaced by alternate combinations of dots having different droplet volumes[0104]. According to above referenced document, the image processing for all the pixels in the image is the same. This makes the method unsuitable for correcting correlated image artifacts that result from systematic variations between nozzles.
The document US2004/00117595 teaches an example of what is known to a person skilled in the art as “color vector error diffusion”. Referring to FIG. 6 in this document, a module 205 quantizes an error corrected color vector. An error calculator 206 calculates the difference between the error corrected color vector before and after quantization, and sends the result to an error diffuser 307. The error diffuser adds a portion of the error back to the next uncorrected color vector to obtain a new error corrected color vector. According to above referenced document, the image processing for all the pixels in the image is the same. This makes the method unsuitable for correcting correlated image artifacts that result from systematic variations between nozzles.
This means that an alternative approach is needed to suppress the image quality artifacts, such as banding or streaking, that result from systematic variations between nozzles belonging to the same or different print heads that are part of a print head assembly. Preferably, the alternative approach does not require processing of the image of a document that is to be printed prior to halftoning and does not require controlling the individual voltages that drive the transducers of nozzles belonging to the same or a different print head.